1. Field of the Invention
The present invention relates generally to heat exchangers and methods of constructing heat exchangers.
2. Discussion of the Background
Heat exchangers and heat exchange chemical reactors having large arrays of parallel tubes are known in the art. Traditional design practices for such articles are codified in design standards. It is known that flow leakage bypassing the flow passages outside the surfaces of the tubes, commonly referred to as the “shell side” of the “tube and shell” exchanger, limits design thermal performance significantly.
Various techniques are used to advantageously increase heat transfer area per unit volume in heat exchangers, such as the use of tubes with extended heat transfer surfaces and the use of an especially-closely packed array of tubes. Such configurations are of important in the construction of compact, cost-effective heat exchange structures. However, the use of such configurations exacerbates the problem of flow bypassing in tube and shell heat exchangers. Therefore, the heat exchanger industry has attempted to limit the effects of flow bypassing by decreasing pressure drop through the flow passages by spacing the tubes far apart and by providing little or no extended heat transfer surfaces (often referred to as fins), which decrease the compactness and cost-effectiveness of the heat exchanger. Alternatively, the heat exchanger industry has attempted to limit the effects of flow bypassing by providing sealing elements to limit leakage in any given flow passage outside the tubes. However, these methods of limiting flow bypassing have several severe limitations.
The method described in U.S. Pat. No. 2,595,822, to Uggersby (hereinafter “the '822 patent”), provides elastic elements formed from metal that possess a rounded outer shape. Such elements are limited to tubular heat exchange arrays having a rounded plan form, such as those referred to as tube and shell exchangers. Further, these elements are limited in their ability to seal against surfaces of high roughness or local surface imperfection. The '822 patent describes a method that is relatively impractical, since many tube and shell exchangers have a rounded shell manufactured by welding rolled plates, and thus local irregularities can only be removed by difficult and/or costly machining or grinding. In many cases, due to the physical size or material of construction, it would be completely impractical to improve the surface finish enough to utilize the method described in the '822 patent. Finally, the metal elements of the invention in the '822 patent are limited to applications below the temperature where creep deformation begins. In fact, even utilizing the metal elements at operation temperatures that are high enough to stress-relieve the metal elements will render them substantially less effective in providing sealing. Thus, temperatures above 400° C. are completely out of the question, and temperatures above 200° C. may cause partial loss of function over long exposures.
An alternative method to that described in the '822 patent is described in U.S. Pat. No. 4,733,722, to Forbes et at. (hereinafter “the '722 patent”). The '722 patent describes elastic elements fabricated from polymer material with a specially designed shape. These sealing elements overcome the problem regarding the sensitivity to surface finish in the elements of the '822 patent. However, the elastic elements described in the '722 patent have even more severe temperature limits than the elastic elements described in the '822 patent.
The problems of limiting bypass flow using seals are made worse in heat exchangers with exceptionally high local pressure gradients in the flow passages outside the tubes. Examples of exchangers of this type are multi-pass, U-tube heat exchangers designated as shell type F in the Tubular Exchanger Manufacturers Association (TEMA) standard nomenclature. The design standards recognize the need for seals in such exchangers, and improved inter-pass seals are described in U.S. Pat. No. 4,778,005, to Smith (hereinafter “the '005 patent”). The improved seals described in the '005 patent are elastic metal elements, which are actively loaded by the gas differential pressure. Such seals still suffer the drawbacks of the circular seals described in the '822 patent to some extent, but benefit by their active nature.
TEMA standard nomenclature does not even recognize exchangers having different shell side passes within a shell which is not longitudinally divided. This indicates the inability of prior art methods to prevent deleterious leakage in such designs. U.S. Pat. No. 6,497,856 to Lomax et al. (hereinafter “the '856 patent”) describes a heat exchange chemical reactor employing an array of tubes and multi-pass flow outside those tubes. In a heat exchange reactor structure of the type revealed by the '856 patent, maximum temperatures between the fluid passages outside the tubes is above 800° C., and thus too hot to employ the method described in the '822 patent. The burner required in the apparatus described in the '856 patent can create a significant pressure drop across the partition between the flow channels. This pressure drop significantly increases deleterious flow bypassing using conventional construction techniques. Further, the apparatus described in the '856 patent is specifically meant to be enhanced in performance by the provision of extended heat transfer surfaces, thus further increasing pressure drop and leakage within the heat exchange reactor.
It is therefore desirable to provide a heat exchange structure that reduces shell-side fluid leakage and bypass for tubular heat exchangers, such as those operated at high temperatures and pressures.